Adds ai_recipe_generation sample (#2242)

Adding the demo app from my I/O talk. Because AI.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

ai_recipe_generation/lib/services/gemini.dart

@@ -0,0 +1,58 @@
+import 'package:google_generative_ai/google_generative_ai.dart';
+
+import '../features/prompt/prompt_model.dart';
+
+class GeminiService {
+  static Future<GenerateContentResponse> generateContent(
+      GenerativeModel model, PromptData prompt) async {
+    if (prompt.images.isEmpty) {
+      return await GeminiService.generateContentFromText(model, prompt);
+    } else {
+      return await GeminiService.generateContentFromMultiModal(model, prompt);
+    }
+  }
+
+  static Future<GenerateContentResponse> generateContentFromMultiModal(
+      GenerativeModel model, PromptData prompt) async {
+    final mainText = TextPart(prompt.textInput);
+    final additionalTextParts =
+        prompt.additionalTextInputs.map((t) => TextPart(t));
+    final imagesParts = <DataPart>[];
+
+    for (var f in prompt.images) {
+      final bytes = await (f.readAsBytes());
+      imagesParts.add(DataPart('image/jpeg', bytes));
+    }
+
+    final input = [
+      Content.multi([...imagesParts, mainText, ...additionalTextParts])
+    ];
+
+    return await model.generateContent(
+      input,
+      generationConfig: GenerationConfig(
+        temperature: 0.4,
+        topK: 32,
+        topP: 1,
+        maxOutputTokens: 4096,
+      ),
+      safetySettings: [
+        SafetySetting(HarmCategory.harassment, HarmBlockThreshold.high),
+        SafetySetting(HarmCategory.hateSpeech, HarmBlockThreshold.high),
+      ],
+    );
+  }
+
+  static Future<GenerateContentResponse> generateContentFromText(
+      GenerativeModel model, PromptData prompt) async {
+    final mainText = TextPart(prompt.textInput);
+    final additionalTextParts =
+        prompt.additionalTextInputs.map((t) => TextPart(t)).join("\n");
+
+    return await model.generateContent([
+      Content.text(
+        '${mainText.text} \n $additionalTextParts',
+      )
+    ]);
+  }
+}